Method for removing cationic contaminants from beverages

ABSTRACT

Process for production of anionically charged filter media sheet including pretreatment of filter elements with cationic charge modifier, preferably employing inorganic colloidal silica charge modifiers. The resulting filters are used for the removal of haze or haze formers from beverages.

BACKGROUND OF THE INVENTION

This invention relates to methods for the preparation of filter mediaand more particularly, to the provision of anionically charged media ofenhanced wet strength from anionic filter elements.

The filtration of fine particle size contaminants from fluids has beenaccomplished by the use of various porous filter media through which thecontaminated fluid is passed. To function as a filter, the media mustallow the fluid, commonly water, through, while holding back theparticulate contaminant. This holding back of the contaminant isaccomplished by virtue of the operation, within the porous media, of oneor both of two distinctly different filtration mechanisms, namely (1)mechanical straining and (2) electrokinetic particle capture. Inmechanical straining, a particle is removed by physical entrapment whenit attempts to pass through a pore smaller than itself. In the case ofthe electrokinetic capture mechanisms, the particle collides with asurface face within the porous filter media and is retained on thesurface by short range attractive forces.

With the exception of microporous polymeric membranes, the porous filtermedia known to the art as being suitable for the filtration of fineparticle size contaminants are comprised of fiber-fiber orfiber-particulate mixtures formed dynamically into sheet by vacuumfelting from an aqueous slurry and then subsequently drying the finishedsheet. In those fibrous filter media that depend upon mechanicalstraining to hold back particulate contaminants, it is necessary thatthe pore size of the filter medium be smaller than the particle size ofthe contaminant to be removed from the fluid. For removal of fine,submicronic contaminant particles by mechanical straining, the filtermedia need have correspondingly fine pores. Since the pore size of sucha sheet is determined predominantly by the size and morphology of thematerials used to form the sheet, it is necessary that one or more ofthe component materials be of a very small size, such as small diameterfibers. See for example, any of Pall U.S. Pat. Nos. 3,158,532;3,238,056; 3,246,767; 3,353,682 or 3,573,158.

As the size of the contaminants sought to be removed by filtrationdecreases, especially into the submicron range, the difficulty andexpense of providing suitably dimensioned fiber structures for optimumfiltration by mechanical straining increases. Accordingly, there isconsiderable interest in the use of fine particulates such asdiatomaceous earth.

However, for such materials it is necessary to provide a matrix in orderto present a coherent handleable structure for commerce and industry.Thus, at least one of the component materials in the sheet is a long,self-bonding structural fiber, to give the sheet sufficient structuralintegrity in both the wet "as formed" and in the final dried condition,to allow handling during processing and suitability for the intended enduse. Unrefined cellulose fibers such as wood pulp, cotton, celluloseacetate or rayon are commonly used. These fibers are typicallyrelatively large, with commercially available diameters in the range ofsix to sixty micrometers. Wood pulp, most often used because of its lowrelative cost, has fiber diameters ranging from fifteen to twenty-fivemicrometers, and fiber lengths of about 0.85 to about 6.5 mm.

In addition to controlling the dispersion characteristics (and thereforethe porosity of the sheet) and providing wet strength, charge modifiersare employed to control the zeta potential of the sheet constituents andmaximize performance in the electrokinetic capture of small chargecontaminants. In practice, cationic charge modifiers are employed sincemost naturally occurring contaminant surfaces are anionic at fluid pH ofpractical interest. Thus, a melamine-formaldehyde cationic colloid isdisclosed for filter sheets in U.S. Pat. Nos. 4,007,113 and 4,007,114.

Biological fluids present a specialized problem in that certain naturalsubstances, commonly of proteinaceous character, are typically presentin the system and are preferably, and even preferentially, removed inthe course of a filtration operation. Unlike the submicronic impuritiesmore typically encountered in other systems, these materials arecationic in nature at applicable pH values, i.e., below the isoelectricpoint for such contaminant.

The surfaces of such filter elements as diatomaceous earth, cellulosefiber and the like may also be characterized as weakly anionic innature, hence it might be expected that these materials would naturallyprovide the desired electrokinetic properties for enhanced capturepotential of the cationic impurities. However, even to take advantage ofthis somewhat limited effect it is necessary to provide a coherentintegral filter medium of controlled, uniform porosity comprised ofinterengaged filter elements, ordinarily requiring a binding agent foradequate wet strength. Also, higher levels of anionic charge are desiredfor optimization of the electrokinetic capture mechanism. The use offilter media comprising binders or charge modifiers in filter systemswith biological fluids poses special problems, among them thepossibility of introducing to the fluid impurities resulting from lossof or a breakdown in filter elements. While certain levels of particularimpurities may be tolerable in some systems, organic extractables poseespecially sensitive problems in the filtration of foods andpharmaceutical products. In filter systems composed of cellulose fiberas a matrix for particulate filter aids modified with an organic chargemodifying resin, organic extractables are naturally primarily traceableto the resin. Selection of the charge modifying resin can alleviate theproblem, even under relatively stringent conditions of use includingsanitization and sterilizable procedures. Even in the absence ofmeaningful levels of extractables, however, many resins of choice aresubject to discoloration in use, tending to limit their marketabilityfor food and drugs.

Further, even low levels of certain organic extractables areunacceptable in some systems, and accordingly it is desirable for thisreason and that of aesthetics to wholly remove the organic chargemodifier resin from the filter construction. At the same time, it isdesirable for the removal of submicron charged contaminants to retainthe charge potential afforded by a charge modifying resin.

The surfaces of the filter elements may be treated with an inorganiccharge modifier such as anionic colloidal silica, but by reason of therepulsive effect of these commonly charged materials, only a modestamount of charge modification is effected, and a coherent structure ofadequate wet strength may not be conveniently prepared, even at highlevels of charge modifier.

U.S. Pat. No. 3,253,978 to C. H. Dexter & Sons Inc. describes a methodof preparing a porous, inorganic sheet product of high strength, free oforganic binders, composed of inorganic fibers or flakes, e.g., glass ormineral wool bound with colloidal silica in which a cationic agent,e.g., cationic starch, is added to the aqueous slurry containing theanionic colloidal silica binder shortly before deposit upon an inclinedFourdrinier wire. No cellulose containing systems are employed. Thepatentee compares performance to the similar use of dicyandiamideformaldehyde condensates with cellulose or asbestos in U.S. Pat. No.3,022,213, evidencing the slow drainage rates experienced. It isprobable that the slow drainage rates are a result of the mutualcoagulation of the anionic colloidal silica and the cationic starch.

It is accordingly an object of the present invention to provide chargemodified filter media sheets of enhanced filtration performance,especially for the removal of submicron contaminants from aqueoussystems at high efficiency.

Another object is to provide charge modified filter media characterizedby low organic extractables over a wide range of filtration conditions.

A still further object is the provision of filter media effective acrossthe spectrum of biological liquids and, particularly, ingestables suchas food and drugs.

A specific object is the provision of anionically charged media ofenhanced wet strength from anionic filter elements.

These and other objects are achieved in the practice of the presentinvention as described hereinafter.

GENERAL DESCRIPTION OF THE INVENTION

The method of the invention involves the utilization of an inorganiccationic charge modifier in the treatment of cellulose pulp andparticulate filter aid, to reduce or reverse the surface charge of theanionic filter elements, whereby anionic charge modification may beeffected to a desired level, in one or more stages. For example, inorder to reduce the repulsive effect of the anionic character of thecellulose fiber or particulate filter aids to the deposition of anioniccharge modifier, sufficient cationic charge modifier may be deposited onat least one of said filter or particulate to reduce or reverse thelatent charge in affected regions or sites thereon to permit depositionof a level of inorganic anionic charge modifier to provide enhancedelectrokinetic charge potential and adequate wet strength. The processmay be iterated as desired to accomplish a selected level of chargemodification.

The filter media sheet is preferably formed by vacuum felting of theanionically disperse aqueous slurry comprising beaten cellulose fibersand fine particulate to provide a uniform, high porosity, and fine poresize structure with excellent filtration and flow characteristics. Thefilter media, comprising cellulose fiber as a matrix, and particulatefilter aid, the surfaces of at least one of which have been modifiedwith inorganic anionic colloidal silica, are free of extractables, suchas formaldehyde or amines originating with organic resinous chargemodifiers, and are free of discoloration, such that the sheets areusable under any sterilizing conditions and may be employed safely andeffectively with potables or ingestables such as food or drugs; andexhibit a wet strength of at least 2.5 kg/in.

The filter sheets so prepared may be used in the treatment of fluids forthe removal of submicronic impurities therein, alone or in conjunctionwith other filter media. In one such combination, the anionicallycharged medium of the present invention may be employed in concert, asin tandem, with a cationically charged medium, as disclosed in copendingand commonly assigned application Ser. No. 027,568 of Hou, et al., nowabandoned, for the efficient removal of differentially chargedcontaminants. A representative use, for removal of hazes and hazeprecursors in alcoholic and fruit beverages is described in copendingand commonly assigned application Ser. No. 065,258 of Green, et al.filed Aug. 9, 1979, now abandoned, incorporated herein by reference.

The inorganic anionic colloidal silica is an aqueous dispersion ofnegatively charged colloidal particles, as disclosed in any one of U.S.Pat. Nos. 2,224,325; 2,285,477; 2,574,902; 2,577,485; 2,597,872;2,515,960; 2,750,345; or 2,573,743; incorporated herein by reference,and available commercially for example as Ludox LS, MS and HS, allaqueous sols containing about 30 percent solids sold by E. I. duPont deNemours and Co.; and Cab-O-Sil, a colloidal silica powder sold by CabotCorporation of Boston, Mass.

While the principal requirement for this component of the filter systemis that it functions as a charge modifier and dispersion agent, itshould also be able to interact with the matrix to provide strongbonding as by cross-linking. It is surprising, in that the bonding oforganic to inorganic materials is more difficult to achieve, to findthat strong bonds are achieved with cellulose fiber upon curing withcolloidal silica. However, the bonding once achieved is relatively moreinert to its surrounding environment, and loss of the material byhydrolytic or solvolytic action, believed to be among the causes ofimpurities or extractables generated in other systems, is obviated. Themaximization of wet strength, even at low levels of cationic modifier,is believed to be related to the essential structural identity of theinorganic cationic and anionic modifiers employed.

In preferred embodiments of the invention, relatively high loadings offine particulates such as diatomaceous earth or perlite, to 50-70percent of more by weight of the sheet, are employed. Without wishing tobe bound by an essentially hypothetical elucidation, it is believed thatsurface modification of these materials with silica colloid,particularly at these high loadings, contributes to the integrity of theoverall structure, and may be attributable to the formation of somesiliceous, or inorganic interbondings, interengaging the relatively lowlevel (10-20%) of cellulose fibers comprising the total sheet weight insuch embodiments, with the particulates by way of the cross-linkingaction of the active hydroxyl sites provided by the colloidal silica.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further illustrated by the accompanyingdrawings, in which:

FIG. 1, is a graph of weight percent of anionic charge modifier vs. wetstrength for the resulting anionically modified sheet, at varying levelsof cationic charge modifier employed in the pretreatment of the filterelements.

FIG. 2, is a graph of normalized streaming potential vs. time, comparingequilibrium flush out curves for a prior art filter sheet, and a filtersheet manufactured in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The filter media sheets of the invention are prepared from anionicallymodified filter elements, usually in the form of an anionically disperseaqueous slurry comprising cellulose fiber and optimized levels of fineparticulate such as diatomaceous earth or perlite. The filter elementsmay be anionically modified in the slurry and the sheet prepareddynamically by vacuum felting, and drying, or the filter elements may bepretreated and formed into sheet media. A special feature of theinvention is the provision of filter media sheet in which the level ofparticulate retained is enhanced as compared to sheet preparedconventionally.

The state of refinement of a wood pulp fiber is determined by means of a"freeness" test in which measurement as the flow rate through a formingpad of the fibers on a standard screen is determined, most commonlyutilizing the "Canadian Standard Freeness Tester". In this method, thequantity which is measured is the volume of water (expressed in ml.)which overflows from a receiver containing an orifice outlet at thebottom. The Canadian Standard Freeness measurements are employed in thepresent specification. Coarse unbeaten wood pulp fibers produce highdrainage rates into the receiver from the screen resulting in largeoverflow volumes, and hence record a high freeness. Typical wood pulpsshow Canadian Standard Freeness values ranging from +400 ml. to +800 ml.In paper or filter media manufacture, such pulps may be subject tomechanical refining processes such as beating which tends to cut and/orfibrillate the cellulose fibers. Such beaten fibers exhibit slowerdrainage rates, and, therefore, lower freeness.

In accordance with the present invention, such beaten pulp is preferablyemployed in the self-bonding matrix for the filter media. The CanadianStandard Freeness of the pulp system will vary with pulp selection, andmay be reflective of varying states of subdivision or refinement, aswhere different pulps or differently beaten pulps are combined for sheetformation, but the beaten pulp will be employed to provide a compositeor average value ordinarily ranging from 100 to 600 ml., with lowervalues e.g., 200-300 ml. or less being preferred for higher solidsretention.

The wood pulp may comprise as little as 10 percent by weight with up to20 to 30 percent, by weight of the total, being preferred to providefilter media sheet with structural characteristics suitable forindustrial filtration applications.

Performance is enhanced by maximizing the amount of fine particulate inthe filter media sheet. While as little as 10 percent of a fineparticulate will result in noticeable improvement in filtrationperformance of either type of media, optimum performance is achieved byutilizing the maximum amount of fine particulate. For industrialfiltration, structural characteristics suggest a practicable maximum ofabout 70 percent by weight. Of course, for less demanding applications,somewhat higher levels will be possible. Generally, levels of 50-70percent by weight are employed.

There are various types of fine anionic particulates that are suitablefor the intended purpose, including diatomaceous earth, perlite, talc,silica gel, activated carbon, molecular sieves, clay, etc. Functionally,the fine particulate should have a specific surface area in excess ofone square meter/gram and/or particle diameters of less than 10 microns.In a broad sense, any fine particulate may be suitable (such as J. M.Filter Cel, Standard Syper Cel, Celite 512, Hydro Super Cel, Speed Plusand Speedflow; Dicalite 215 and Dicalite 416 and Dicalite 436) and maybe evaluated by techniques well-known to the art. Siliceous materialsare preferred, and from the standpoint of size, morphology, cost, fluidcompatibility and general performance characteristics, the finer gradesof diatomaceous earth/perlite for example, in proportion by weight offrom about 80/20 to 20/80 give better filtration performance or bettercost/performance characteristics than that achieved by the use of anysingle type by itself. Similarly, mixtures in all proportions ofrelatively coarse and fine particulates, e.g., 50/50 parts by weight of10 and 5 micron diameter particulates may be used.

In paper production, where charge modifiers are sometimes used, theobjective is reduction of charge to approximately the isoelectric pointto maximize efficiency in interfelting of fiber. For filtration, maximumcharge is desired to enhance removal of charged particles byelectrokinetic mechanisms. In the present case the surface charge of atleast one of the negatively charged filter elements, i.e., cellulose andparticulate is first reduced to render the surface less electronegativeand receptive to deposition of the desired amount of anionic chargemodifier, whereupon the surface is rendered even more electronegativeproviding at least certain more highly electronegative regions or siteswithin the filter sheet.

In the first stage of the process, the particulate filter aid and beatencellulose pulp is dispersed in an aqueous medium, and treated with aninorganic cationic charge modifier, preferably cationic silica colloid.

Suitable cationic colloidal silica materials include those of U.S. Pat.Nos. 3,007,878; 3,252,917; 3,620,978; 3,719,607; and 3,956,171 allincorporated by reference.

These are aqueous dispersions of negatively charged colloidal particlesconsisting of a dense silica core coated with a negatively chargedpolyvalent metal-oxygen compound, e.g., of the class consisting of metaloxides, metal hyxroxides and hydrated metal oxides of metals having avalence of 3 or 4, preferably aluminum and titanium. Most preferably thedispersion is acidic, and the coating is of polymeric alumina species.Typically, the mole ratio of aluminum to silica on the surface is about1:1, and the dispersion (which has been commercially available as LudoxPositive Sol 130M, from E. I. duPont de Nemours & Co.) is stabilizedwith a counterion, as described in the aforesaid U.S. Pat. No.3,007,878. The dispersion has been supplied at 30% solids stabilizedwith chloride ion (1.4%, as NaCl) for use in the pH range 3.5 to 5.5.

The colloidal particles exhibit a surface area of about 150-225 m² /g bynitrogen adsorption, a particle diameter of about 15-16 mu, and amolecular weight of about 5 to 18 million by light scattering.

In its preferred form, the characteristics of the silica aquasol arefurther modified to higher levels of polymeric alumina speciescalculated as alumina from 13 to 15% or more, in the stable range basedupon the colloidal solids. The coating, or overcoating, may be achievedby simply treating with an appropriate aluminum compound, e.g., basicaluminum chloride, as described in U.S. Pat. No. 3,007,878, or anothersource of polybasic aluminum cations. The alumina in such systems existsas a surface coating and, to the extent it exceeds the available surfacearea, as free alumina in solution. The free alumina may, of course, alsoserve as a coating for virgin filter elements, e.g., particulate presentand systems so prepared offer improved resistance to autoclaving and hotwater flushing conditions together with added wet strength. Whendesired, the resulting colloidal dispersion may be, and customarily istreated to remove excessive electrolyte, as by dialysis, in order toachieve storage stability.

The amount of inorganic cationic colloidal silica employed in theinitial treatment of the filter elements is in general an amountrendering the surfaces of the cellulose pulp and/or particulatereceptive to the deposition of the inorganic anionic colloidal silica toa level providing enhanced electrokinetic capture potential forpositively charged submicronic contaminants and adequate wet strengthi.e., at least 2.5 kg/in. in the sheet. Preferably, an amount ofinorganic cationic colloidal silica just sufficient to modify thesurface charge of the filter elements electroneutral or slightlyelectropositive is employed. Usually a relatively small amount ofinorganic cationic charge modifier in the range of 0.1 to 1.0 wt.percent by weight of the filter elements proves sufficient. This issomewhat surprising, considering the surface area involved, but may beexplicable in the sense of providing a minimum number of receptivebonding sites for the anionic charge modifier. Of course, larger amountsof inorganic cationic modifier may be used but, given the objective ofproviding an anionically charged sheet, are contraindicated since thiswill necessitate employing correspondingly larger amounts of inorganicanionic modifier.

The inorganic cationic charge modifier is essentially fully sorbed ontothe surfaces of at least one of the cellulose pulp and particulatefilter aid within a very short period of time i.e., essentially withinone minute, whereupon the second stage treatment may be commenced. Itwill of course be understood that the filter elements may be pretreatedwith inorganic cationic charge modifier, and then modified anionicallyin the sheet forming slurry.

In this stage, employing the inorganic anionic charge modifiersdescribed above, the amount of charge modifier employed is preferablythat sufficient to at least provide an anionically disperse system,i.e., a system in which no visible flocculation occurs at ambientconditions in the absence of applied hydrodynamic shear forces. Thesystem therefore comprises essentially discrete fiber/particulateelements exhibiting a negative charge or zeta potential relativelyuniformly or homogeneously distributed in and throughout the aqueousmedium. The specific level will, of course, vary with the system and themodifier selected but will be readily determined by one skilled in theart.

The charge modification effected is demonstrable in measurements ofsurface zeta potential, and in improved filtration efficiency forpositively charged particles in liquid systems.

The slurry of pulp and particulates is formed in any suitable manner.The sequence of adding these components to water to form the initialslurry appears to be relatively unimportant, except that, as aforesaid,the inorganic cationic charge modifier is essentially fully sorbed,deposited or coated onto the filter elements prior to addition of theinorganic anionic modifier. The consistency of the slurry will representthe highest possible for a practical suspension of the components,usually about 4 percent. The system is subjected to hydrodynamic shearforces as by a bladed mixer, and the charge modifier is then added tothe slurry.

The shear level is not critical, i.e., any otherwise suitable shear rateor shear stress may be employed having regard for available equipment,preferred processing times etc. but is selected and employed simply tobreak up the flocs as required and maintain the system in a dispersedcondition during treatment. Of course, upon the formation of ananionically disperse slurry, the system is free of floc formation evenin the absence of applied shear.

After charge modification, the slurry is diluted with additional waterto the proper consistency required for vaccum felting sheet formation,ordinarily 0.5 to 21/2 percent, depending upon the type of equipmentused to form the sheet, in a manner known to the artisan. The slurry isformed into a sheet and oven dried in standard manner. The performanceof the sheet as related to the drying parameters and optimizedconditions may reflect energy considerations or desired thermal historyconsistent with minimization of unnecessary exposure to elevatedtemperatures, especially as the decomposition or scorch point for thesystem is approached.

In accordance with a preferred embodiment of the invention, filter mediasheets are formed from filter elements, i.e., particulate and aself-bonding matrix of beaten cellulose pulp, at least one of which ischarge modified, the pulp being a system incorporating beaten pulp toprovide a Canadian Standard Freeness of up to 600 ml., preferably lessthan 300 ml. e.g., 100-200 ml. the charge modifier consisting ofinorganic anionic silica and being applied in a proportion to enhanceelectronegativity of the surface and secure a wet strength of at least2.5 kg/in. Filter media sheets so prepared may be autoclaved, hot waterflushed or otherwise treated at elevated temperature to sanitize orsterilize the structure.

Most preferably, both cationic and anionic charge modifiers comprise thesame basic silica material which is believed to contribute to themaximization of performance characteristics, especially wet strength.

While the invention has been principally described by reference tofilter media sheet, it will be understood that the principles set forthwill have similar applicability to the construction of other fiber andfibre-particulate structures including depth filters in wound orcompacted form.

Filter media sheets in accordance with the invention may be employedalone or in combination with other such media to treat fluids containingproteinaceous contaminants wherein the pH of the fluid is ofsufficiently low value so that the proteinaceous contaminants areessentially cationic. Specifically, such filter media sheets areeffective in the removal of submicron protein chill hazes which areformed in distilled spirits, beers, fruit juices and other such low pHgenerally acidic fluids.

The present invention is representatively illustrated in the followingexamples, in which certain tests are performed as described hereinafter.

Oil Flow Test

As a measure of the porosity of the filter media sheets, 100 ssu oil ispumped through the sample sheet until a differential pressure drop of 5psid is attained, at which point the flow rate (ml./min.) is recorded.

Wet Strength

Wet strength is determined via a tensile test on a 2" wide test specimenwhich has been presoaked in distilled water for 5 minutes, employing UTMChatillon Model UT-SM.

Normalized Streaming Potential

The measurement of streaming potential is a conventional means ofdetermining zeta potential i.e., the electric potential excess of thesurface, and the surrounding fluid to the hydrodynamic shear plane, overthe bulk potential of the fluid. In the present test, streamingpotential values are determined, and normalized for differing pressuredrop in the media being tested, expressing the results in units ofmillivolts per foot of water. The filter media is evaluated by flushingout the filter media with water until the measured streaming potentialachieves a relatively stable maximum value. At this point, the filtermedia has ceased to contribute any significant ionic species to thewater, i.e., the inlet resistivity equals the outlet resistivity.

The filter media test cell is based on the design of Oulman, et al.JAWWA 56:915 (1964). It is constructed from Lucite having an effectivearea of 3.14 square inches (2" diameter) and is equipped with platinumblack electrodes. Water and mercury manometers are used to measure thepressure drop across the media being evaluated. Streaming potentialvalues (by convention, of opposite sign to the zeta potential and thesurface charge) are measured with a high impedance volt meter. Theinfluent and effluent resistance are monitored with conductivity flowcells (cell constant=0.02/cm) using a resistance bridge.

Upon the attainment of equilibrium streaming potential (i.e., afterflush out) contaminant challenge tests may be performed in the samesystem. The tests above are described in more detail in a paperpresented at the 71st Annual AICHE meeting (1978): "Measuring theElectrokinetic Properties of Charged Filter Media", Knight, et al.

In the following examples which further illustrate this invention;proportions are by weight, based upon total pulp and particulate,excluding charge modifier.

EXAMPLE I

In each of the following runs, cellulose pulp having an average CanadianStandard Freeness of 130 ml. (in an amount to constitute 31% by weight)was dispersed in water to a consistency of about 4%, 426 perlite(diatomaceous earth supplied by Grefco, having a mean particle size of4.2 microns) was added (in an amount to constitute 69% by weight) whilemaintaining the consistency with the addition of water, and theinorganic anionic silica colloid charge modifier added, while the systemwas maintained under agitation (hydrodynamic shear applied by action ofa Hei-Dolph stirrer by Polyscience Inc., having 4 propeller blades,rotating at about 700 ppm on setting 2). The slurry was subsequentlydiluted to 0.5 percent consistency and vacuum felted into a sheetranging from about 0.160 to 0.200 inch thickness (depending uponretention) in a nine inch by twelve inch hand sheet apparatus utilizinga 100 mesh screen. The sheet was subsequently removed, dried in a staticoven at 350° F. until constant weight was achieved, and the final weightrecorded.

To demonstrate the effect of pretreating the filter elements withinorganic cationic silica colloid, in runs 5-11 the slurry was firstdispersed with the cationic silica colloid, and the system agitated for15 minutes to effect deposition of the modifier on surfaces of theparticulate/fiber components, whereafter anionic silica colloid wasdispersed in the system, deposited on the filter elements and the sheetforming completed as above.

The vacuum filtered sheets were compared in respect of flow propertiesand wet strength as follows:

                  TABLE I                                                         ______________________________________                                               Pulp    Anionic   Cationic                                                    Free    Silica    Silica Oil     Wet                                   Sheet  ness    Colloid,  Colloid,                                                                             Flow    Strength                              No.    (CSF)   Wt. %     Wt. %  (ml/min.)                                                                             (kg/in.)                              ______________________________________                                        Control                                                                              130     0         0              1.20                                  1.     130     3         0      21.0    2.15                                  2.     130     5         0      20.0    2.30                                  3.     130     6         0      18.0    2.60                                  4.     130     10        0      15.0    2.75                                  5.     130     5         0.1    20.0    3.27                                  6.     130     5         0.2    18.0    3.60                                  7.     130     6         0.2    18.0    3.65                                  8.     130     5         0.4    18.0    4.00                                  9.     130     3         0.1    --      2.6                                   10.    130     6         0.1    --      3.3                                   11.    130     3         0.2    --      2.7                                   ______________________________________                                    

As will be seen from the foregoing results, graphically represented inaccompanying FIG. 1, pretreatment with the inorganic cationic colloidalsilica prior to treatment with the inorganic anionic colloidal silicaenhanced the wet strength of the sheet considerably, while maintainingcomparable flow rates.

EXAMPLE II

The procedure of Example I was repeated except that the pulp freenesswas modified under constant conditions, as follows:

                  TABLE II                                                        ______________________________________                                               Pulp    Anionic   Cationic                                                    Free-   Silica    Silica Oil     Wet                                   Sheet  ness    Colloid,  Colloid,                                                                             Flow    Strength                              No.    (CSF)   Wt. %     Wt. %  (ml/min.)                                                                             (kg/in.)                              ______________________________________                                        1.     241     6         0.1    42.5    2.35                                  2.     130     6         0.1    18.0    2.60                                  3.     62      6         0.1    12.5    3.25                                  4.     22      6         0.1    7.0     3.45                                  ______________________________________                                    

The lower freeness values (more highly beaten pulp) were preferred forwet strength, but affected flow. From the results of this and the priorexample, it can be seen that lower levels of anionic silica colloid,e.g., 5% with higher levels of cationic silica colloid, e.g., 0.3% arepreferred for the best balance of wet strength and flow.

EXAMPLE III

In this Example, the performance of a prior art cationic silica colloid(Wesol PA) charge modified filter media sheet (see copending applicationSer. No. 027,568) was compared to filter media sheet prepared fromfilter elements pretreated with cationic silica colloid, and then chargemodified with anionic silica colloid in accordance with the invention.

A. Filter media sheets were prepared containing 30% by weight of acellulose pulp system (C.S.F. about 130) and 70% by weight ofparticulate (The filter sheet of the invention employed a mixture ofperlites, and whereas that of the prior art utilized a 50/50 admixtureof diatomaceous earth and perlite) and were each formed in identicalmanner by preparing an ionically disperse aqueous slurry chargemodifying, vacuum felting and oven drying, except that in the case ofthe invention, the filter elements were first dispersed with cationicsilica colloid (Wesol PA, 0.1%) for a period of 15 minutes sufficient topermit essentially complete deposition, and thereafter the filterelements were charge modified with anionic silica colloid (Ludox HS-30,5%) whereas in accordance with the prior art the filter elements afterdeposition of cationic modifier (Wesol PA, 6%) thereon.

Employing test conditions detailed above, normalized streaming potentialvalues were determined over time, and equilibrium flush out curvesplotted for the respective filter media, compared in FIG. 2. As will beseen, the prior art media exhibits an increasingly negative normalizedstreaming potential which stabilized at a high equilibrium valveindicative of a high positive surface charge. The media prepared inaccordance with the invention exhibits a positive normalized,equilibrium streaming potential which corresponds to its anionic nature.

B. The anionic filter media sheet of the invention was subjected tocontaminant challenge in a manner illustrative of the removal ofsubmicronic proteinaceous substance. Blended whiskey containing anabundant well dispersed very fine haze (8 NTU, initial pH, 4.1 ambienttemperature) was passed through the anionic filter media. Effluentturbidity values were reduced to about 1 NTU, evidencing the removal ofcationically charged haze formers. Similar results were achieved withunstable rum, and other liquids containing proteinaceous impurities ofcationic nature below the isoelectric point.

While the invention has been described hereinabove with reference to theuse of a single filter sheet media, it will be understood that multiplesheets may be used to provide further depth, or the anionic filter sheetmay be used in coordination, as in tandem with a cationic filter sheetto remove differentially charged impurities. In a preferred embodimentthis invention constitutes an improvement to the process of stabilizingunstable beverages against haze development disclosed and claimed incopending and commonly assigned application Ser. No. 065,258 of Green,et al., filed Aug. 9, 1979, now abandoned, in which the beverage isconducted through a first filter medium, the surfaces of which aremodified with a polyamido polyamine epichlorhydrin cationic resin,forming haze in the beverage, the improvement comprising thereafterconducting the beverage through the anionic filter sheet of theinvention to remove haze or haze formers. Hazes formed in such beveragesby the conventional chilling process may of course also be removed inthis manner.

What is claimed is:
 1. A process for the removal of haze or haze formerswhich are cationic submicronic proteinaceous contaminants from abeverage, comprising conducting a beverage containing such contaminantsthrough a filter media having an anionic electrokinetic capturepotential for such contaminants, the filter media being prepared by:a.treating filter elements selected from the group consisting of cellulosepulp and particulate filter aids with an amount of inorganic cationiccolloidal silica surface charge modifier to render the surface of thefilter elements receptive to the subsequent deposition of an inorganicanionic colloidal silica charge modifier, thereafter, b. chargemodifying the treated filter elements with an inorganic anioniccolloidal silica charge modifier, and c. forming the filter elementsinto a sheet having a negative zeta potential and a wet strength of atleast 2.5 kg/in.
 2. The process of claim 1, wherein the amount ofcationic charge modifier is at least sufficient to render the surfacecharge of the filter elements electroneutral.
 3. The process of claim 1,wherein the cationic charge modifier is employed in an amount of fromabout 0.1 to about 1.0 weight percent of the filter elements.
 4. Theprocess of claim 1 or 3, wherein the anionic charge modifier is employedin an amount of from about 1 to 10% by weight of the filter elements. 5.The process of claim 1, wherein the particulate filter aid is siliceous.6. The process of claim 1, wherein the haze or haze formers are producedby chilling the beverage.
 7. The process of claim 1, wherein the haze orhaze formers are produced by conducting the fluid through a cationicfilter media.
 8. The process of claim 7, wherein the cationic filtermedia has the surface thereof modified with a polyamido polyamineepichlorhydrin cationic resin.